A PORTAL TO THE PAST: TIMING OF SEDIMENT UNDERPLATING AT PORTAL RIDGE, CALIFORNIA

Subduction of sediments at convergent margins influences coupling at the plate interface, geochemical cycling, and geodynamic response of the overall margin. In areas of shallow subduction, sediments may be accreted to the base of the overlying plate. In western North America, a series of underplated sediments known as the Pelona-Orocopia-Rand schist (PORS) are exposed across southern California and western Arizona. The southern cluster of PORS exposures have been used to argue for a wide zone of shallow sediment underplating during the Cretaceous based on compiled metamorphic conditions and the transition from detrital to metamorphic zircon U-Pb ages at ca. 73 Ma. A parallel study on the northern cluster of Rand and related schists are needed to better understand the regional and temporal variations in sediment underplating. In this work, we present new petrographic and zircon U-Pb data on the westernmost exposure of PORS that has not been translated or rotated due to Neogene strike-slip faulting associated with the San Andreas transform system. The lithologies at Portal Ridge are dominantly quartzofeldspathic schist with graphitic albite porphyroblasts, biotite, and muscovite, as well as rare garnet and tourmaline. Less common compositions include mafic amphibolites characterized by hornblende, plagioclase, and local garnet and minor occurrences of garnet-bearing quartzites. Metamorphic mineral assemblages record upper greenschist to amphibolite facies. We analyzed zircon grains from eight samples with U-Pb geochronology to characterize the source of the quartzofeldspathic protolith and look for metamorphic overgrowths. We applied depth-profiling to capture thin outer growth zones based on the characteristics documented at PORS exposures in western Arizona. Preliminary data shows age zonation in 10–20% of grains analyzed. We will differentiate detrital from metamorphic ages using geochemical signatures to constrain the age of metamorphism, and, if a robust population of metamorphic ages are present, report an interpreted age of sediment subduction based on the transition from detrital to metamorphic ages. Finally, we will compare our new data from the northern PORS to southern Orocopia schist exposures to understand spatial and temporal variations in the timing of sediment underplating.